Dust-laden and sometimes toxic exhaust gases build up during various production processes in a steel plant. Estimates assume that only 15 kilograms of dust are created per ton of steel produced in an electric arc furnace. If one assumes that a medium-sized steel plant produces 400,000 tons of steel a year, each year 6000 tons of dust emissions would therefore accrue.
In order to protect the workers and the environment, it is important to suction away the accruing dust emissions at the point of origin. This suctioning must guarantee that the work place limit values (WLV) of dust-laden emissions are observed. The observance of these values is realized by so called dust removal systems. Dust removal systems are divided into primary and secondary systems. The primary dust removal is responsible for disposing of the dust emissions building up directly at the machines during the melting of steel scrap (e.g., at the electric arc furnace). The secondary dust removal is responsible for disposing of the dust emissions occurring in the production bay. This is done by hood designs situated in proximity to the respective dust source (e.g., at the converter). Such a dust removal system [consists] of three main components, namely, a pipeline network, an induced draft fan, and a filter chamber.
In order to suction away the dust emissions at the respective place of origin, defined volumetric flow rates are needed at the suctioning points. The magnitude of these suctioning volumetric flow rates depends on how heavy is the concentration of the dust emission in the surrounding air and it is determined by a visual estimation during the operation of the system. By a visual estimation is meant the manual adjusting of the volumetric flow rates at the corresponding suction points. For this, during an actual production process the volumetric flow rate is increased at the corresponding suction points until the estimate by the naked eye reveals that all exhaust gases are being transported away by the volumetric flow rate. A measurement of the (WLV) value in the vicinity of the production process is done to verify that the adjusted volumetric flow rate is sufficient. If so, the relevant operating parameters for this (such as partial vacuum in the main channel) are plotted and factored into the control system of the dust removal process.
In order to adjust the required volumetric flow rates at the different suction points, single-vane or (in the case of larger pipe diameters) multiple-vane exhaust air flaps are installed in the pipelines leading to a suction point, which can travel between 0 and 100% in their closed position. The required partial vacuum of a dust removal system is produced by the induced draft fan. This basically consists of two to three key components, namely, an impeller, an electric motor, and possibly a hydraulic coupling or a frequency converter. The electric motor places the impeller in a rotatory movement. The impeller, based on the geometrical arrangement of the vane wheels, then assures a partial vacuum in the pipeline network. This pressure difference between the surrounding pressure and the partial vacuum created in the pipeline network ensures that a volumetric flow rate is produced in the direction of the induced draft fan.
The required delivery power of such a system is defined by the following equation.Pzu={dot over (V)}zuΔPμes 
It is clear from this formula that both the suctioning volumetric flow rate {dot over (V)}zu and the total pressure loss Δμges have major impact on the required delivery power Pzu of a secondary dust removal system. For example, if one accepts the practically common values for the suctioning volumetric flow rate of 2,000,000 m3/h of air subjected to dust removal, and assumes that this volumetric flow rate is produced with a pressure difference of 50 mbar, a required delivery power of around 2.78 MW would result for such a dust removal system. This example should show on the one hand how energy-intensive secondary dust removal systems are, and on the other hand point out that it is important to reduce system pressure losses to a minimum.
Patent document EP 0116 727 A2 describes a feedback control method for a dust removal system in which each suctioning point is associated with a feedback control circuit with individual adjustable nominal value. Furthermore, there is an overarching feedback control circuit for the induced draft fan. The feedback control process requires special sensors, which are complicated and costly.
The problem to be solved by the present invention is to disclose a method in the form of a new control concept which enables an energy-efficient operation of secondary dust removal systems and overcomes at least some of the aforesaid drawbacks. Moreover, a device as well as a computer program should be disclosed which implement the method according to the invention. Since an uneconomical operation of secondary dust removal systems at a time of worldwide increasing energy prices is no longer sustainable by the operators of these plants, there is an increasing demand to optimize dust removal systems in terms of energy input.